Highly efficient photon detectors play a major role in countless applications in physics, nuclear engineering and medical physics. In nuclear engineering, radioactive waste can be characterized with photon detectors using nondestructive assay techniques (PNDA). In medical physics, photon detectors are extensively used for diagnostic x-ray and CT imaging, nuclear medicine, and quite recently, radiation therapy of cancer.
In radiation therapy of cancer, ever more accurate delivery techniques spur the need for efficient detectors for million electron volt (MeV) photons in order to allow the imaging of the patient during radiation delivery. In particular, in Tomotherapy, a detector for MeV photons can be used for both the CT imaging and for verifying the dose received by the patients.
Referring now to FIG. 1, an ionization detector 10 may be used for the detection of radiation in the thousand electron-volt (KeV) range such as is used in conventional diagnostic x-ray and CT. The ionization detector 10 employs a set of conductive laminae 12 oriented generally along an axis 14 of the propagating radiation. The lamina 12 may be spaced apart along a transverse axis generally parallel to the radiation axis 14 in parallel configuration defining between them detector volumes 16. The detector volumes 16 may be filled with a gas having a high atomic number, such as xenon, which may be further pressurized to increase the density of xenon atoms within the detector volume 16.
An incident KeV x-ray 18 entering the detector volume 16 will have a high probability of colliding with a xenon atom (not shown) to create one or more secondary electrons 20 within the detector volume 16. These electrons 20 produce negatively and positively charged ions within the detector volume 16. The height of the detector volume 16 along the radiation axis 14 may be adjusted so that substantially all KeV x-rays 18 entering the detector volume 16 will experience one such collision.
Opposite laminae 12 surrounding the given detector volume 16 are biased with a voltage source 21 causing the migration of the ionization charge to the oriented lamina 12. The current generated by such electron flow is measured by a sensitive ammeter circuit 22, providing an indirect measure of the amount of incident KeV radiation 18.
The laminae 12 thus first serve as collector plates for the ionization detector 10. They also serve to block oblique KeV radiation 18′ scattered by the intervening patient from being imaged thus improving the sharpness and clarity of the image. The laminae 12 further serve to prevent migration of the electrons 20 between detector volumes 16 such as would produce cross talk further blurring the image. The laminae 12 are optimized in thickness in the transverse direction consistent with these roles.
The ionization detector 10 of FIG. 1 would not be expected to be efficient for MeV x-rays which would be expected to pass fully through any practical thickness of xenon, generating relatively few electrons.
Referring now to FIGS. 2a and 2b, more efficient detection of MeV x-rays 24 may be accomplished by the use of a converter plate 26 which converts the MeV x-rays into high-energetic charged particles which are subsequently recorded electronically or photonically. In a first embodiment of FIG. 2a, a detector 25 uses a converter plate 26 that is an opaque, high density, high atomic number material, such as lead, placed above detector media 28 to convert each photon of MeV x-rays 24 into multiple electrons 20. The detector media 28 may be film, an ionization-type detector 10, a scintillation detector or other well-known detector types.
A high atomic number and/or high-density material is preferred for the converter plate 26 because it has a high cross-section for the interaction of high-energy photons. Generally, however, the height 30 of the converter plate 26 is limited to less than that required to filly absorb the MeV x-rays 24 correspondingly limiting the conversion efficiency of the detector 25. The reason for this is that increasing the height 30 to provide for more absorption of MeV x-rays becomes fruitless as additional ejected electrons are balanced by increased absorption of electrons within the converter plate 26 itself.
Referring to FIG. 2b, the limitation imposed by the converter plate 26 of detector 25 of FIG. 2a, may be overcome by using a transparent scintillating converter plate 26′ as shown in FIG. 2b. Here the MeV x-rays 24 striking the scintillating converter plate 26′ produce photons 34 which pass through the transparent scintillating converter plate 26′ to be received by light detector 36. The transparent scintillating converter plate 26′ may be made thick enough to block a greater proportion of the MeV x-rays 24 because the mobility of photons within the transparent scintillating converter plate 26′ is proportionally much greater than the mobility of electrons within the solid converter plate 26. Transverse movement of the photons within the transparent scintillating converter plate 26′ may be blocked by opaque elements 38 which may, for example, be slices cut into the material of transparent scintillating converter plate 26′ and filled with a light and x-ray blocking material so as to define regular detection areas.
Ideally the scintillating material will have a relatively high atomic number and great transparency. Unfortunately, the manufacture of transparent scintillating converter plate 26′ using such high quality scintillators is significantly more expensive than the manufacture of conventional converter plate 26 shown in FIG. 2a and the efficiencies of such radiation detectors remain modest.
What is needed is a relatively simple, inexpensive, and high efficiency radiation detector suitable for high-energy radiation.